U.S. Pat. No. 4,225,989 to Corbett et al. disclosed a ripple bed having an upper and a lower inflatable layer. The upper layer has separate air passages inflatable and deflatable to provide the rippling effect, and the lower layer is separately inflatable to provide support over the area of the upper layer when deflated. However, once the lower layer is broken such as pricked by an article of sharp end, the whole bed shall be invalidated and may be abandoned to waste money. The lower layer is formed by the upper and lower sheets to be a complete pad which may not be well bent, thereby being unsuitable for adjusting any bending position for a hospital mattress.
A dual layer cellular inflatable pad of U.S. Pat. No. 3,674,019 disclosed by Grant includes a first inflatable cellular layer having a plurality of cells for inflation, and a second cellular cushioning layer alternately interfitted with the cells of the first inflatable cellular layer in tangential relationship with each other. In order to prevent a relative shear movement of the Grant pad, each upper cell should be positioned in between two lower cells in tangential relationship therebetween for a stable supporting of the upper layer by the lower layer. Therefore, a precise production and quality control is required to maintain the tangential contacts between the upper cells and the lower cells of the upper and lower layers, possibly causing production complexity and increasing fabrication cost therefor. Meanwhile, once the cellular structure of either upper layer or lower layer is broken, the whole pad will be leaked and may be completely abandoned to waste money and influence environmental protection.